The present invention relates to a glass keyboard and to a method for producing a glass keyboard.
Such pressure switch elements are known as touch panels in displays. The touch panels are normally made from transparent plastic films whose inner surfaces are coated with an electroconductive material. To support said films spacers are glued in the air gap, wherein a spacer arranged around the outside of the contact area is bonded in an airtight manner to the plastic films to stabilize the inside air pressure thus supporting the upper film. Inside the contact area elastic spacers are additionally provided which ensure return movement of the films. It is a drawback of the known pressure switch element that it requires a hermetically sealed air space which does not allow for any pressure compensation. In the event of considerable deviation from the normal atmospheric pressure, e. g. during application in submarine vehicles or in aeronautics and space operations, and at high temperatures hairline cracks occur in the vapour-deposited electroconductive contact layer due to the changes in air pressure, which results in a failure of the unit. At large heights the spacer in the contact area expands. This changes the switching path of the contact film, and the given electronical and mechanical parameters, such as the action point, are no longer complied with. Further drawbacks of the known touch panels are that the plastic films present only a limited mechanical and chemical resistance, a small degree of transmission and are not antistatic. Further, in the event of temperature variation there is the danger of crack formation in the electroconductive layer due to fact that the expansion coefficients of the conductive layer and the plastic carrier considerably differ from each other.
From EP 0 546 003 B1 a pressure switch element made from a glass laminate is known which comprises a flexible thin glass pane and at least one carrier glass pane, each pane being provided with an electroconductive layer on the faces facing each other. The opposing electroconductive layers are kept at a distance to each other with the aid of a spacer. The electroconductive layers touch each other when pressure is applied to the flexibel thin glass layer at the essentially localized place of pressure load application.
The known pressure switch element comprises a shock and pressure-sensitive thin glass pane.
It is the object of the invention to provide a glass keyboard with reduced pressure and shock sensititivity thus allowing large-scale production. According to the invention the flexible thin glass pane is preferably made up of a drawn thin glass film. It is important that the thin glass pane is produced from a drawn thin glass film since only this type of pane is sufficiently stable and flexible at the same time. The drawn thin glass film thus allows production of a break-proof thin glass pane which is flexible enough to allow contact to be established between the electroconductive layers facing each other.
According to an alternative embodiment the carrier material pane is slightly larger than the flexible thin glass pane such that the marginal area of the carrier material pane protrudes beyond the the marginal area of the thin glass pane. The protruding marginal area of the carrier material pane protects the sensitive marginal edge of the thin glass pane, which can further reduce the danger of breakage of the thin glass pane.
In the marginal area the flat set back margin of the thin glass pane is glued to the marginal area of the carrier material pane by means of an adhesive acting as a spacer. The protruding margin of the carrier material pane is further adapted to receive an adhesive build-up which also protects the sensivitve edge of the thin glass pane.
Preferably the margin of the thin glass pane is stabilized by means of a cured plastic material. The cutting edge of the thin glass pane displays a plurality of microcracks which occur during the cutting process and extend from the edge to the inside. Said microcracks may easily result in a crack which destroys the entire thin glass pane. The margin of the thin glass pane is therefore preferably stabilized by means of a cured plastic material. For this purpose the boundary edges of the thin glass pane are dipped into a liquid plastic material. Due to the capillary effect the microcracks are filled with the liquid plastic material whereafter the plastic material cures. When the plastic material is cured, the thin glass pane offers a considerably higher stability since breaking of the thin glass pane starting from its margins can no longer occur in the event of pressure or shock load.
The keyboard surface made from the flexible thin glass pane may be of flat configuration in the marginal area and present a-slight convexity to the out-side in the keyboard area. The convexity of the keyboard surface improves on the one hand the return movement behaviour of the thin glass pane after operation and prevents on the other hand the occurrence of Newton""s rings which are undesired in a glass keyboard for optical reasons.
A thin glass pane is preferably glued to the carrier material pane in the deep-drawn state of the former. For this purpose the thin glass pane is deep drawn in cold condition and in this condition glued to the carrier material pane such that the convexity of the keyboard is maintained.
The thickness of the thin glass pane ranges between approximately 0.1 and 0.5 mm, preferably between approximately 0.175 and 0.4 mm. A thin glass pane of such a thickness offers an adequate flexibility to allow for localized switching contact between opposing electroconductive layers.
The spacer is arranged exclusively in the marginal area of the keyboard surface between the thin glass pane and the carrier material pane, wherein in the remaining portion of the keyboard surface switching operations can be performed at any location without further spacers being provided. The invention preferably makes additional spacers in the area of the switching section superfluous such that the overall keyboard surface is available for switching operations without any limitations.
In a preferred embodiment the spacer in the marginal area is made from a plastic material cured under UV-light. This offers the advantage that no separate spacer has to be provided, and that the spacer can already be formed when the thin glass pane is glued to the carrier material pane.
The carrier material pane is preferably transparent. This allows e. g. keyboard letterings and/or illumination means to be provided behind the carrier material pane.
Preferably the carrier material pane is made of glass. This offers the advantage that the expansion coefficients of a carrier material pane made of glass and of the electroconductive layer, e. g. an indium tin oxide layer, display only minor differences such that the danger of crack formation in the electro-conductive layer is reduced.
According to a preferred aspect of the invention the carrier material pane is made from a transparent glass cell with integrated electroluminescent matrix (EL display glass).
The carrier material pane can be made from multilayer glass to ensure protection against splintering. Such a glass keyboard can preferably be employed in areas where there is the danger of explosion or vandalism. The use of multilayer glass further offers the advantage of a cut-off effect against UV-light.
The carrier material pane may comprise a heating means on the side averting the thin glass pane.
The heating means preferably is an electroconductive transparent coating applied to the carrier material pane.
The carrier material pane may also comprise an electroconducting shielding layer on the side averting the thin glass pane, said layer shielding electromagnetic perturbing radiation occurring behind the glass keyboard, e. g. from the electronic control unit, thus reducing the susceptibility of the glass keyboard to electromagnetic interference.
The transparent carrier material pane may form an optical filter or be provided with such a filter.
According to another aspect of the invention the carrier material pane comprises a receiving means for an exchangeable sheet for keyboard lettering on the side averting the thin glass pane. Such a receiving means is e. g. configured as an insertion pocket for the exchangeable sheet. The exchangeability of the sheet for keyboard lettering allows the functions of certain keyboard fields to be fixed individually or to be subsequently changed. In the case of machine control it is thus possible to e. g. assign the keyboard fields on the keyboard surface to different functions according to requirement.
The carrier material pane and/or the spacer comprise in the marginal area vent openings for the space between the thin glass pane and the carrier material pane. This offers the advantage that pressure compensation is possible in the event of deviation from the normal atmospheric pressure, e. g. during application in submarine vehicles or in aeronautics and space operations, and at high temperatures such that the electroconductive layers on the thin glass pane and the carrier material pane are prevented from being damaged.
The vent openings are preferably provided with a filter material protecting the glass keyboard from soiling.
The carrier material pane may comprise a light-scattering layer on the side averting the thin glass pane. Such a coating or surface treatment is advantageous e.g. in the case of background illumination.
The thin glass pane may comprise a decorative margin on its lower side. The decorative margin offers the advantage that on the one hand the keyboard surface to be operated is limited whereby the switching section to be operated is easily recognizable and on the other hand larger production and installation tolerances are available for means located behind the pane, such as LCD displays, which may decrease the production costs.
Further preferred features of the invention are stated in the subclaims.
Hereunder embodiments of the invention are explained in detail with reference to the drawings in which: